There is considerable interest in using rare earth doped fiber amplifiers to amplify weak optical signals for both local and trunk optical communications networks. The rare earth doped optical amplifying fibers are low in cost, exhibit low noise, provide relatively large bandwidth which is not polarization dependent, display substantially reduced crosstalk problems, and present low insertion losses at the relevant operating wavelengths which are used in optical communications. Contemplated rare earth doped optical fiber amplifiers can be coupled end-to-end to a transmission fiber and coupled, through a directional coupler, to a laser diode pump. The directional coupler is designed to have a high coupling ratio at the pump wavelength and a low coupling ratio at the signal wavelength so that maximum pump energy is coupled to the amplifier with minimal signal loss. When the amplifying medium is excited with the pump laser, signal light traversing the amplifier experiences a gain. The pump energy may be made to propagate either co-directionally or contra-directionally relative to the signal energy, depending upon whether any remaining unconverted pump light can be more conveniently filtered at the transmitter or the receiver.
A particular area of current concern is that of increasing the capacity of an existing optical fiber communication system. Presently, this can be done by either increasing the bit rate or by adding wavelength division multiplexed (WDM) channels. Recent advances in erbium-doped fiber amplifier technology suggests that increased capacity can best be obtained with WDM channels.
However, a major problem in implementing WDM channels on an optical transmission system is the absence of "gain equalization". More specifically, because of the nonuniform wavelength-dependent gain profile and saturation characteristics of optical fiber amplifiers such as erbium-doped fiber amplifiers, each channel in a WDM system will experience a different optical gain which, in turn, can result in an excessive bit-error rate for channels with low gain. Substantial efforts are presently being expended in developing components which will equalize the powers of WDM channels at each optical amplifier repeater. This invention is directed toward providing a simple, relatively inexpensive solution to the problem of increasing the capacity of an erbium doped optical fiber lightwave system which does not require either new equipment or adjustments at intermediate optical amplifier sites.